Study of [Fe(pyim)3] complexes: dynamic spin equilibrium on encapsulation in zeolite Y

Tris complex of Fe^II2(2′-pyridyl)imidazole has been encapsulated in the supercages of zeolite Y and characterized by using powder XRD, FTIR, Mössbauer spectroscopy, variable temperature magnetization and MAS NMR techniques and results have been compared with those obtained for this complex with ClO₄⁻ and SO₄²⁻ as anions. At room temperature, the Fe(pyim)₃](ClO₄)₂ complex exhibited low spin state, while Fe^II(pyim)₃]SO₄ exhibited the existence of both low and high spin states. The encapsulated Fe^II(pyim)₃]²⁺ complex exhibited a broad quadrupole doublet characterized by isomer shift, δ=+0.55 mm/s and quadrupole splitting ΔE_q=1.26 mm/s. The magnetization measurements carried out for the encapsulated Fe^II(pyim)₃]²⁺ complex showed a systematic decrease in its values with decreasing temperature down to 75 K with no indication of thermal hysteresis effects. These results suggest the existence of a dynamic spin state equilibrium between the high and low spin states for the encapsulated Fe^II(pyim)₃]²⁺ complex with time constant comparable to the characteristic Mössbauer time scale of ⁵⁷Fe nuclei.